1. Field of the Invention
The present invention provides spoke geometry for a non-pneumatic tire that is less prone to fatigue when used. In particular, the spoke geometry is provided with an optimized thickness profile over the length of the spoke. This optimization results in a reduction in the peak strain energy density levels in the spoke, thereby reducing the likelihood of crack initiation and propagation which in turn enhances the durability of the spoke and tire.
2. Description of the Related Art
Non-pneumatic or structurally supported tires have been disclosed in the art. For example, U.S. Pat. No. 7,201,194, commonly owned by the applicant of the present invention, relates to a structurally supported resilient tire that supports a load without internal air pressure. The content of this patent is hereby incorporated by reference in its entirety. In an exemplary embodiment, this non-pneumatic tire includes an outer annular shear band and a plurality of web spokes that extend transversely across and radially inward from the annular band and are anchored in a wheel or hub. In certain exemplary embodiments, the annular shear band may further comprise a shear layer, at least a first membrane adhered to the radially inward extent of the shear layer and at least a second membrane adhered to the radially outward extent of the shear layer. In addition to the ability to operate without a required inflation pressure, the invention of U.S. Pat. No. 7,201,194 also provides advantages that include a more uniform ground contact pressure throughout the length of the contact area. Hence, this tire mimics the performance of a pneumatic tire.
FIG. 1 shows such a tire, defining a radial direction R and transverse direction T (which is perpendicular to the radial direction and perpendicular to the direction in which the tire rolls). For reference, all the reference numerals in the 100's used herein refer to a previous tire and spoke design while all reference numerals in the 200's used herein refer to a new and improved tire and spoke design according to an embodiment of the present invention. The tire 100, 200 comprises a tread 102, 202 that is attached to the outward extent 104, 204 of the spokes 106, 206, which in turn, are connected to a hub or wheel 108, 208 at their inward extent 110, 210 by means known in the art. For the version of the tire 100, 200 shown, the spokes 106, 206 are formed by pouring a polyurethane liquid into a mold, such as a rotational mold, where the liquid is then cured or hardened. It can also be seen that the spokes 106, 206 are grouped in pairs and that the individual spokes 106′, 106″, 206′, 206″ within each pair are consistently spaced from each other and that each pair is spaced consistently from the adjacent pair around the circumference of the tire. The spacing within each pair and the spacing between each adjacent pair do not need to be the same.
As described by the Abstract and col. 2, lines 28-41 of the '194 patent, the spokes 106, 206 support the tire 100, 200 in tension near the top of the tire 100, 200 and not in compression. Instead, the spokes 106, 206 at the bottom of the tire near the contact patch, which is where the tread 102, 202 of the tire contacts the road, compress or buckle easily. This helps the tire to simulate the pneumatic support function of a pneumatic tire. As can be imagined, these spokes 106, 206 undergo a great deal of cyclic stress from tension to compression especially as the tire 100, 200 rotates at high speeds. This creates a risk of fatigue failure for the spokes. Consequently, the endurance of the spokes 106, 206 and the operability of the tire 100, 200 depend significantly on the geometry with which the spokes 106, 206 are made.
Looking now at FIG. 2 a side sectional view of a previous spoke design that is susceptible to fatigue failure caused by crack initiation and propagation is shown. For the sake of clarity, only the spokes are shown. The thickness of the spoke, T106, is relatively consistent at 2.8 mm, and the ends 112′, 112″ of the spokes 106′, 106″ where the spokes connect to the hub and tread are about 1 mm thicker at 3.8 mm. Hence, there is a transition area at either end of the spoke where the thickness of the spoke tapers down so that at roughly 25% of the radial height Hr of the spoke from either end, the spoke is at the ultimate reduced thickness of 2.8 mm.
Testing of this spoke design has revealed that the fatigue limits of the spoke limits the load capacity of the tire. If the load is too great, then the tire a tendency over time to develop cracks that eventually propagate as the spoke 106 cycles between tension and compression as the tire 100 rolls on a road surface. Accordingly, there is a need for an improved spoke design that reduces the likelihood that crack initiation and propagation will occur at higher load limits. Also, such a design that can be easily molded and that does not degrade other tire performances would be particularly beneficial.